1. Field of the Invention
The present invention relates to a metal gasket assembly for a cylinder head, which is interposed between a cylinder block and a cylinder head of an internal combustion engine.
2. Related Art
For example, Japanese Patent Laid-Open No. H11-6564 discloses a metal gasket assembly for a cylinder head, this conventional metal gasket assembly comprises a metal base plate and a metal reinforcement plate having a thickness smaller than that of the base plate and lapped with the latter.
Further, in this conventional metal gasket assembly, the base plate is formed therein with cylinder openings formed in positions corresponding to cylinder bores of a cylinder block of an internal combustion engine, annular beads formed around the cylinder openings and having cross-sectional shape of xe2x80x98Vxe2x80x99, a plurality of coolant holes formed around the annular beads and an outer peripheral bead formed so as to entirely surround the annular beads and the plurality of coolant holes and having a cross-sectional shape of slope. The reinforcement plate comprises fold-over parts which are folded around the peripheral edge parts of the cylinder openings in the base plate toward the projection side of the annular beads and extend up to the inside of the annular beads so as to form grommets embracing the peripheral edge parts, lapped parts extending from the peripheral edge parts of the cylinder openings in the base plate to the outer edge part of the base plate on the depression side of the annular bead, and reinforcement beads formed on the lapped parts and projected in the same direction as the direction of the projection of the annular beads so as to be made into close contact with the annular beads, stress relieving parts formed also on the lapped parts and adapted to expand and contract at the positions of coolant holes in the base plate.
However, in this conventional metal gasket, since the reinforcement plate extends up to the outer edge part of the base plate, beyond the coolant holes in the base plate, the reinforcement plate is lapped with the outer peripheral bead of the base plate so as to bear a fastening force of fastening bolts for a cylinder head, and accordingly, the surface pressure of the annular beads is lowered. Thus, there has been raised such a problem that the sealing function of the annular beads on the base plate cannot be enhanced further.
Further, Japanese Patent Laid-Open No. H11-2325 discloses a metal gasket assembly for a cylinder head, and this conventional metal gasket assembly comprises a metal base plate, and a reinforcement plate having a thickness smaller than that of the base plate, and lapped with the base plate.
Further, in this metal gasket assembly, the base plate has cylinder openings formed at positions corresponding to cylinder bores in a cylinder block of an internal combustion engine, annular beads formed around the cylinder openings and having a cross-sectional shape of xe2x80x98Vxe2x80x99, a plurality of coolant holes formed around the annular beads, and an outer peripheral bead formed so as to entirely surround the annular beads and the coolant holes and having a cross-sectional shape of slope. The reinforcement plate comprises fold-over parts which are folded onto the projection side of the annular bead around the peripheral edge parts of the cylinder openings in the base plate, and extend into the inside of the annular bead so as to form grommets embracing the peripheral edge parts, and lapped parts which extend from the peripheral edge parts of the cylinder openings to positions outside of and vicinity of the annular beads, on the depression side of the annular beads.
However, since the above-mentioned conventional metal gasket assembly has the flat reinforcement plate, a combustion pressure in the cylinder bores of the cylinder block should be sealed by the resilient force of only the annular beads on the base plate, and accordingly, there has been raised such a problem that the sealing function cannot be enhanced sufficiently for the combustion pressure.
An object of the present invention is to provide a cylinder head metal gasket assembly which can advantageously solve the above-mentioned problems, and accordingly, the metal gasket according to the present invention comprises a metal base plate, and a metal reinforcement plate having a thickness smaller than that of the base plate and lapped with the base plate, the base plate having cylinder openings formed at positions corresponding to cylinder bores in a cylinder block of an internal combustion engine, annular beads formed respectively around the cylinder openings and having a V-shaped cross section, a plurality of coolant holes formed in the peripheral parts of the annular beads, and an outer peripheral bead entirely surrounding the annular beads and the plurality of coolant holes and having a cross-sectional shape of slope, and the reinforcement plate having fold-over parts which are folded onto the projection side of the annular beads around the peripheral edge parts of the cylinder openings in the base plate, and extend into the inside of the annular beads so as to form grommets embracing the peripheral edge parts, lapped parts which extend from the peripheral edge parts of the cylinder openings to positions in the vicinity of the annular beads and outside of the annular beads, on the depression side of the annular beads, and reinforcement beads formed in the lapped parts and projected in the same direction as the direction of the projection of the annular beads so as to be made into close contact with the annular beads.
This cylinder head metal gasket assembly is interposed between a cylinder block and a cylinder head of an internal combustion engine, and is then fastened so that the annular beads formed around the cylinder bores corresponding respectively to cylinder bores in the cylinder block, and having a cross-sectional shape of xe2x80x98Vxe2x80x99, and the reinforcement beads which are projected in the same direction as the projecting direction of the annular beads so as to be made into close contact with the annular beads, can prevent combustion gas from blowing to the outside from the cylinder bores of the cylinder block, through a high resilient force in cooperation of the annular beads and the reinforcement beads, and the outer peripheral bead which entirely surrounds the plurality of coolant holes corresponding to coolant passages formed respectively in the cylinder head and the cylinder block and communicated with one another, can prevent coolant from leaking from the coolant passages.
Further, with this cylinder head metal gasket assembly, the lapped parts of the reinforcement plate extend only to positions outside of the annular beads and around the annular beads in the base plate, but do not extend to the outer edge part of the base plate, beyond the coolant holes in the base plate, and accordingly, the reinforcement plate is prevented from being lapped with the outer peripheral bead of the base plate in order to prevent a fastening force of a cylinder head fastening bolt from being born. Therefore, the surface pressure of the annular beads becomes higher enough, and further, the annular beads of the base plate and the reinforcement beads of the reinforcing plate are lapped with one another so as to seal the outer peripheral edges of the cylinder bores with a high resilient force in cooperation between the annular beads and the reinforcement beads. The grommets formed by the reinforcement plate restrain the degrees of depression of the annular beads and reinforcement beads, and accordingly, the resilient forces of the beads can be prevented from lowering.
Accordingly, with the cylinder head metal gasket assembly according to the present invention, the sealing function against the combustion pressure in the cylinder bores can be sufficiently enhanced, and the high degree of the sealing function can be maintained for a long time.
In a preferred embodiment of the present invention, a second base plate having the same planar shape and thickness as those of the base plate but having annular beads and an outer peripheral bead which are projected in a direction different from the direction of the projection of the annular beads and the outer peripheral bead of the base plate, is lapped with the base plate on the projection side of the annular beads thereof. With this arrangement, the sealing function can be further enhanced by these beads on both base plates.
Further, in a preferred embodiment of the present invention, the reinforcement plate is made of a material having a degree of elongation which is greater than that of the base plate. With this arrangement, when the reinforcement plate is folded so as to form grommets, the fold-over parts can be easily formed with no defect.
Further, in a preferred embodiment of the present invention, the difference in thickness between the base plate and the reinforcement plate is set to be equal to or greater than 50 xcexcm but equal to or smaller than 150 xcexcm. With a difference in thickness smaller than 50 xcexcm, the surface pressure of the annular beads is not sufficiently high, but with a difference in thickness higher than 150 xcexcm, the sealing function of the outer peripheral bead is lowered.